Caption: False-colour, three-dimensional map of part of the East Pacific Rise, showing a discontinuity called an overlapping spreading centre (OSC). The East Pacific Rise (EPR) is a mid ocean ridge lying to the west of Central America. It marks the boundary between the Pacific and Cocos tectonic plates and is a constructive plate boundary or spreading centre where basalt magmas rise to the surface, forming new igneous rock and pushing the older crust away on either side. The OSC comprises two ridge segments, offset by 8km and overlapping by 25km, separated by a 600 metre deep overlap basin. Abandoned ridges and fossil overlap basins surround the OSC. The area shown is 80 x 65km. For more information see Nature, Vol 335, p217-.